High frequency heating system with inductive plasma



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w. GKRTNER April 22,v 1969 HIGH FREQUENCY HEATING SYSTEM WITH INDUC'IIVEPLASMA Filed Nov 14. 1966 i I f I 11 1 VENTOR. WERNER GARTNER AGENTUnited States Patent Int. Cl. HilSb 9/06 U.S. Cl. 219--10.75 3 ClaimsABSTRACT OF THE DISCLOSURE A high frequency plasma induction heatingsystem in which ignition of the gas is produced by means of first andsecond capacitors that form a resonant circuit with the induction coil.The usual priming electrode of prior art systems is thereby'eliminated,

The present invention relates to a high frequency heating system usingionized gas or plasma as the heating medium. More particularly, theinvention relates to an improved ignition circuit for supplying highfrequency energy to the discharge plasma,

In prior art arrangements, it is common practice to connect the inductordirectly to the output of a high frequency generator, in which case thehigh-frequency current passing through the inductor is substantiallyconstant. However, since the reactance of the inductor is very low, thegas ignition has to be initiated by external action, for example, bymeans of a priming arc.

Furthermore, careful attention must be taken during operation of thesystem to prevent excessive variations of the supply of gas. A manualreadjustment of the generator coupling is required in order to avoidde-energization of the overload relay or the extinction of the plasma.For that purpose a variable impedance, for example, an adjustablecapacitor, is connected between the coupling coil and the inductor, asis shown, for example, in the French patent specification 1,371,963,

The initiation of the gas ionization by an external agency and a manualvariation of the coupling are generally required to operate a highfrequency plasma induction heating system. It is an object of theinvention to avoid such complications, In accordance with the invention,a first capacitance is connected in series and a sec-= 0nd capacitanceis connected in parallel with the inductor. The two capacitances, incommon, are tuned with the inductance of the inductor to the resonancefrequency. In this way, a considerably higher reactive power that isalways sufiicient to initiate ignition, and a considerably improvedstabilisation, are obtained.

A variation of the capacitances may be obtained by adding or subtractingfurther capacitances, whereas the inductance of the inductor may bevaried by changing the distance between the turns.

It is especially effective to adjust the working point of the resonantcircuit on the inductive side of the resonance curve, sinceapart fromthe reduction of voltage across the inductor subsequent to ignition dueto the higher 3,440,387 Patented Apr. 22, i969 ice damping etfect, afurther voltage reduction is obtained due to reduction of theinductance,

The nature of the invention will be more clearly apparent from thefollowing description, taken in conjunction with the sole figure of theaccompanying drawing of a preferred embodiment of the invention.

The figure shows a conventional high frequency generator. The highfrequency energy thereof is coupled out by means of the coil 2. The coilis connected to a resonant circuit formed 'by the series capacitance 3,the inductor 4 and the capacitance 5 connected in parallel therewith.The inductor 4 supplies the high frequency energy to the plasma. 1

By appropriate choice of the ratio between the capacitances 3 and 5, themode of operation of the arrangement can be varied. A comparatively highcapacitance 3, for example, provides a rapid action, and a lowcapacitance 3 provides a slow action of the system, An avejrage actionis obtained, for example, when the ratio between the two capacitances isabout 1:1. I

What is claimed is:

1. A high frequency plasma induction heating system wherein theimprovement comprises a stabilized ignition circuit for the systemcomprising, a source of high frequency power of a given frequency, aninduction coil for generating the high frequency plasma field, first andsecond capacitors, means connecting said first and second capacitors inseries and in parallel with said coil, respectively, to form a resonantcircuit therewith that is tuned on the inductive side of the resonancecurve, the resonant frequency of said resonant circuit beingapproximately the same as said given frequency of the high frequencypower source, and means for coupling the output of said high frequencysource to the input of said resonant circuit.

2. A system as claimed in claim 1 wherein said first and secondcapacitors have the same capacitance value,

3. A system as claimed in claim 1 wherein said high frequency powersource comprises an oscillator having an inductor and a capacitorconnected to form a parallel. resonant circuit connected to the outputcircuit of the oscillator, and said coupling means comprises a coilinductively coupled to said oscillator inductor and con= nected to theresonant circuit comprising the induction coil and said first and secondcapacitors.

References Cited UNITED STATES PATENTS 2,662,162 12/1953 Blok um mmmmma: fill-10.75 X 3,277,265 10/1966 Reboux was 1.. 219 121 3,340,4159/1967 De Ruiter et a1 219-421 3,353,060 11/1967 Yamamoto et al. a, 219121 FOREIGN PATENTS 667,657 3/ 1952 Great Britain, 896,399 11/ 1953Germany.

RICHARD M, WOOD, Primary Examiner.

L. H. BENDER, Asistant Examiner.

U.S. Cl, X.R. 219-121 I

